1. Field of the Invention
The present invention relates generally to electrochemical cells and to devices incorporating same, such as fuel cells, sensors, and electrochemical cell gas sensors used for detection of gases in environments containing or susceptible to presence of same, and in a specific aspect, the invention relates to an electrochemical cell utilizing ionic liquid as an electrolyte medium thereof.
2. Description of the Related Art
Gas sensors are used in many commercial and industrial applications, including workplace monitoring for the presence of toxic or otherwise hazardous or deleterious gases and in other applications where health and safety issues require detection of specific gases in the ambient environment.
In these various applications, it is frequently necessary or desirable to monitor concentration of selected gas species down to levels on the order of a few parts per million and less.
Gas sensors used in the foregoing applications include electrochemical gas sensors, which may operate to electrochemically reduce the gas species to be monitored. Alternatively, the gas sensor may operate by electrochemically oxidizing the target gas species sought to be detected. As a still further alternative, the electrochemical gas sensor may operate by indirect oxidation or reduction reaction of a compound that is produced in the gas sensor device, involving the target gas to be detected in the monitored gaseous environment.
Electrochemical gas sensors utilize sensor cells that typically contain three electrodes—the working electrode, the reference electrode and the counter electrode, although gas sensor cells are known having 2-electrode and 4-electrode structures. The electrodes are conventionally mounted within a housing that additionally contains an electrolyte, contacts and electrical wires forming electronic circuitry of the sensor, and a gas permeable membrane that keeps the electrolyte within the cell and allows the gas to contact the measuring electrode.
Electrochemical sensor cells require an electrolyte as a component of the electrochemical cell. The electrolyte performs the transport of electrical charge between the different electrodes and therefore enables an electrical current to flow. The transport of electrical charge by the electrolyte is ionic in character rather than involving charge transport by electrons.
The electrolyte usually is constituted by a liquid solvent containing electrolyte/salt component(s), wherein the solvent comprises water, or alternatively a non-aqueous solvent medium, but the electrolyte may otherwise comprise a solid electrolyte such as yttrium-stabilized zirconia (YSZ) for usage at high temperatures of about 450 to 950° C. or an ion exchange membrane such as a Nafion® membrane (commercially available from DuPont de Nemours and Company, Wilmington, Del.) that is saturated with water.
Liquid electrolytes in some instances can be ‘solidified’ by addition thereto of gel-forming agents, but the resultantly solidified electrolyte materials still rely on the presence of liquid solvent medium for operability, as do ion exchange systems such as the ion exchange membranes mentioned above.
Gas sensors are operationally “open systems” in the sense of requiring gas flow therethrough for detection of gas component(s) in the gas stream with which the gas sensor is contacted. One major deficiency of conventional gas sensors is the limited service life of liquid electrolytes that include gels or wetted membranes, since these materials dry out over time as solvent evaporates therefrom.
Water-based gas sensor systems are designed to be in equilibrium with ambient humidity of the environment being monitored, but their utility is generally restricted to a specific humidity range.
Solid electrolyte gas sensors are difficult to construct for efficient operation, since diffusion of gas in the solid electrolyte material is hindered in relation to liquid electrolytes, there is no transport of reactants in the solid electrolyte, and known solid electrolyte materials tend to be hygroscopic and thus are difficult to stabilize in humid environments. Additionally, solid electrolytes (i.e. solid materials providing intrinsic ion transport by usually oxide ions O2−) need high operating temperatures. For example, yttrium-stabilized zirconia (YSZ) used in oxygen sensors does not provide sufficient ion conductivity at temperatures below 450° C.
The art therefore continues to seek improvements in electrochemical cell gas sensors.